The present invention relates generally to a method for continuous production of (meth)acrylate syrup and (meth)acrylate-based adhesives therefrom.
Adhesives are provided in various forms for application. For example, adhesives can be prepared and provided in organic solvent for application, after which time the solvent is removed. Adhesives can also be prepared and applied without use of organic solvent—e.g., as in the case of hot-melt adhesives (i.e., where the adhesive is substantially polymerized prior to its application to a substrate) or web-polymerized adhesives (i.e., where the adhesive is substantially polymerized after its application to a substrate). Hot-melt adhesives have a sufficient viscosity upon melting, such that they can be applied to a substrate. Yet, some method of increasing the cohesive strength of applied hot-melt adhesives is often needed (e.g., post-crosslinking or moisture-curing), resulting in decreased processing efficiency and other limitations.
In addition to their various forms, adhesives are used in a variety of applications and are based on a variety of chemistries, which are often dictated by the nature of the desired application. For example, adhesives based on (meth)acrylate chemistry are commonplace, particularly when viscoelastic adhesive properties are desired.
Conventionally, (meth)acrylate-based adhesives are prepared starting with stock (meth)acrylate monomer that is formed according to known methods (e.g., an esterification reaction between an oxo alcohol and (meth)acrylic acid such as that described in U.S. Pat. Nos. 3,776,947; 4,280,009; and 5,386,052). Methods relying on an esterification reaction typically include use of elevated temperatures, reduced pressure, (meth)acrylic acid, an acid catalyst, a molar excess of one component—typically oxo alcohol, and distillation to purify the resulting (meth)acrylate monomer.
Once prepared, processing is generally discontinued and stock (meth)acrylate monomers are stored for later use. Often, polymerization inhibitors (e.g., quinone-based polymerization inhibitors) are present in stock (meth)acrylate monomer in order to prevent premature polymerization thereof during this typical processing delay. An example of the use of quinone-based polymerization inhibitors for this purpose is described in U.S. Pat. No. 3,816,267. Quinone-based polymerization inhibitors are commonly employed, and they can be deactivated by removing oxygen from the system in which they are used (e.g., by purging the system with nitrogen prior to polymerization of the monomer). Thus, special storage and handling conditions are generally associated with stock (meth)acrylate monomer prior to its polymerization.
From compositions comprising stock (meth)acrylate monomer and any other types of monomers and components desired in an adhesive prepared therefrom, adhesives can be prepared using batch or continuous processes. Known continuous processes for preparation of web-polymerized adhesives involve coating of partially polymerized syrup onto a moving web, such as those processes described in U.S. Pat. Nos. 4,181,752; 4,303,485; 4,421,822; and 5,462,977. When continuously preparing (meth)acrylate-based web-polymerized adhesives, it is conventional to form a syrup having a coatable viscosity and based on stock (meth)acrylate monomer. As described in U.S. Pat. No. 4,303,485, that syrup can be conventionally prepared by supplying ultraviolet radiation to a container including stock (meth)acrylate monomer to partially polymerize the same. The step of partial photopolymerization is stopped at any point simply by turning off the ultraviolet radiation. That syrup can then be stored, if necessary, until a point in time that it is used to form an adhesive. When forming an adhesive, for example, the syrup can be coated onto a moving web, for example, where further polymerization thereof can be initiated to form the adhesive.
U.S. Pat. No. 4,303,485 describes a method for producing partially polymerized (meth)acrylate syrup utilizing photopolymerization techniques. The syrup is partially polymerized to a coatable viscosity. In producing the coatable syrup, (meth)acrylate monomer—in the presence of a photoinitiator—is irradiated with ultraviolet radiation for less than about one minute, creating a (meth)acrylate syrup having a viscosity of 0.3-10 Pascal-seconds (300-10,000 centipoise) at ordinary room temperature. The coatable syrup is stated to be a viable article of commerce in that it can be stored for later coating and exposure to ultraviolet radiation in order to convert the coating to its desired end use (e.g., a pressure sensitive adhesive tape).
In contrast to continuous web-polymerized methods, conventional methods of batch polymerization of adhesives and methods of continuous production of hot-melt type adhesives typically involve running of an initial polymerization reaction to at least near complete conversion, and often complete conversion, of the monomer to polymer. In both cases, the adhesive is substantially polymerized prior to its application to a substrate as compared to web-polymerized adhesives (i.e., where the adhesive is substantially polymerized after its application to a substrate).
Limitations of hot-melt processing methods and resulting adhesives are known to those of ordinary skill in the art. Limitations associated with batch processing methods, which often utilize solvents (i.e., water or organic solvents), are also known to those of ordinary skill in the art. Notably, use of solvents with batch processing methods, particularly organic solvents, is undesirable from an environmental, safety, and economic standpoint and is becomingly increasingly more so with time. For example, solvent-based batch polymerization of (meth)acrylates typically requires use of large and costly processing equipment in order to safely dissipate heat arising from the exothermic polymerization reaction. In addition, solvents used during the polymerization reaction must then typically be removed from the polymerized composition. This removal process undesirably increases processing time and cost and poses environmental challenges.
In view of environmental and safety concerns as well as the ever-present desire to optimize production efficiency, alternative methods for production of adhesives and intermediates thereof are desirable. Particularly desired are alternative methods for the continuous production of (meth)acrylate-based web-polymerized adhesives and their intermediates.